[move][doc] some editing of the new section on phantom types
[move][doc] some editing of the new section on phantom types

I noticed a typo in #9263 and decided to do some minor editing of
the surrounding text while I was fixing it. I don't think there are
any substantive changes here.

Closes: #9339

libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core